It is often desirable, for decorative or functional reasons, to apply a coating over a plastic substrate. For certain substrates it has been difficult to find coating compositions that provide the required adhesion at a reasonable price and with suitable physical properties. It is well-known that it is difficult to obtain good adhesion of paints to olefinic substrates, including thermoplastic polyolefin (TPO) substrates and other such modified polyolefin-based materials.
In addition to painted articles, TPO has been used in a color-in-mold process to produce articles of the desired colored that are not intended to be painted, so long as the requirements for appearance are not stringent. The color-in-mold process, however, is not adequate for producing high class ("class 1") surfaces for a number of reasons. First, it is difficult to achieve high gloss finishes, and difficult to control the gloss of the finish reproducibly from one article to the next. Secondly, TPO is relatively soft and it would desirable to coat the substrate with a coating composition that can offer resistance to marring and scratching. Finally, uncoated or unpainted TPO may have unsatisfactory weathering properties (i.e., may degrade, discolor, or chalk during outdoor exposure) in certain applications, whereas it is known that coatings provide good weathering characteristics to many different substrates. For these reasons, then, it would be desirable to have a means of applying a coating composition to an olefinic substrate to provide the properties lacking in the uncoated (raw) uncolored or colored substrate.
Plastic substrates may be coated with curable, or thermosettable, coating compositions. Thermosettable coating compositions are widely used in the coatings art, particularly for high-performance primers and topcoats. Color-plus-clear composite coatings have been particularly useful as topcoats for which exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired.
In the past, it has been necessary to include one or more additional separate manufacturing steps to prepare an olefinic substrate for painting so that the coating layer will be able to adhere to the olefinic substrate. According to one frequently used method, a thin layer of an adhesion promoter or tie layer is applied directly to the olefinic substrate. The desired coating layer or layers are then applied over the adhesion promoter. Such adhesion promoters typically include a chlorinated polyolefin. Adhesion promoters with chlorinated polyolefins are expensive to use, often exhibit instability, and, if used in a clear (unpigmented) composition, produce colored, hazy films. In addition, he chlorinated polyolefin materials may produce coatings with poor exterior durability because they are usceptible to degradation when exposed to UV light. Another method that has been used to prepare an olefinic substrate to receive a coating layer is chemical modification of the substrate surface, for example by flame or corona pretreatment.
Recently, coating compositions that include significant concentrations of adhesion promoting agents in order to achieve good adhesion to olefinic substrates have been proposed. Published international application WO 97/35937 describes a composition that includes 5-45% by weight of resin solids of a substantially saturated polyhdroxylated polydiene polymer having terminal hydroxyl groups. International Publication Number WO 97/35937 and all of the references cited therein are hereby incorporated herein by reference. The international publication '937 discloses that such polymers are the hydrogenated product of dihydroxy polybutadiene produced by anionic polymerization of conjugated diene hydrocarbon capped with two moles of ethylene oxide and terminated with two moles of methanol. (The ethylene oxide produces the oxygenated anion, and the methanol provides the hydrogen cation to form the hydroxyl group.) The large amount of this adhesion promoting agent that must be included may adversely affect physical properties and appearance of the resulting coating. In addition, compositions that include significant concentrations of the adhesion promoting agent may separate into phases because the different components frequently are not very compatible. The '937 reference requires a specific solvent package that may be undesirable in many instances. The same problems are encountered with other prior art adhesion promoting agents such as chlorinated polyolefins. It is also known that including chlorinated polyolefins in some coating compositions, e.g., curable coating compositions that include acid catalysts, can result in adverse interactions between the different components of the coating composition.
It is an object of the present invention to provide a material that can be used to prepare an adhesion promoter or used as an adhesion promoting agent or additive in a coating composition to provide excellent adhesion to uncoated olefinic substrates while overcoming the drawbacks of the compositions and methods previously used.